The instant invention relates to magnesium aluminum silicate (MAS) type structures of low thermal expansion and optimized through-the-wall soot loaded permeability suitable for use in high temperature filtration applications. Specifically, the present invention relates to cordierite wall flow particulate filters having narrower pore size distributions and more interconnected porosity than commercially available cordierite wall flow filters.
A great deal of interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material present in the diesel exhaust stream, which has been declared a toxic air contaminant.
Diesel particulate material is mainly carbon soot. One way of removing the carbon soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel wall flow particulate filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The diesel particulate filter (DPF) is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow. In the industry cordierite has been the cost-effective material of choice for DPFs due to its combination of excellent thermal shock resistance, filtration efficiency, and durability under most operating conditions.
As the layer of diesel particulate materials or carbon soot collects on the surfaces of the inlet channels of the filter, the lower permeability of the soot layer causes a gradual rise in the back pressure of the filter against the engine due to a pressure drop across the length of the filter. This causes the engine to work harder. Once the carbon soot in the filter has accumulated to some level, the filter must be regenerated by burning the soot to restore the back pressure to low levels. The higher the pressure drop across the filter the higher the back pressure against the engine.
For a given filter size and cell geometry, the rate at which pressure drop increases with soot loading depends on how the soot interacts with the pores in the wall of the filter. Commercially available cordierite DPFs designed for truck engines exhibit high pressure drops, necessitating a reduction thereof. It has been observed that filters made from ceramics having well interconnected pores that span a narrow range in pore diameter exhibit a lower pressure drop at a given soot loading than filters that have a broad range in pore size (as determined by mercury porosimetry).
It would be considered an advancement in the art to obtain a cordierite diesel particulate filter with the pore size distribution and pore connectivity capable of maintaining lower pressure drops than has been possible so far with commercially available cordierite DPFs, while still exhibiting low thermal expansion. The present invention provides such a filter and a method of making the same.